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2. The emergence of national governments with the elimination of internecine warfare and the emergence of national markets which permitted a more equitable distribution of the nation's product.
3. Improvements in environmental sanitation and personal hygiene, resulting in uncontaminated food and potable water and a decrease in the probability of infection and contagion.
4. The natural disappearance of some of the agents of disease and death; for example, scarlet fever.
5. The development of modem medicine, climaxed by chemotherapy and the availability of pesticides.
During the modern era, these developments upset the equilibrium between the birth rate and the death rate that characterized most of the millennia of human existence. In the Netherlands in the 1840's, for example, of 1,000 infants born one fourth had died by age 2.5 years and one half by age 37.5 years. In contrast, a century later one fourth had not died until age 62.5 years; and one half, not until 72.5 years (9). As a result of such decreases in death rates, the 100 million Europeans of 1650 had, 3 centuries later, about 940 million descendants.
The areas that are today classed as "developed" or "economically advanced" achieved reductions in mortality throughout the modern period—but mainly since the mid-19th century. In fact, expectation of life at birth has increased more since 1850 than in the preceding 200 years (9). Death rates decreased; birth rates remained at relatively high levels. Therefore, natural increase—the excess of births over deaths—greatly accelerated in the economically advanced regions, primarily Europe, northern America, and Oceania. This was the first population explosion. However, growth rates through natural increase alone rarely exceeded rates of 1.5 percent per annum.
The rapid growth of population in the industrialized nations continued through the 19th century and into the early part of the 20th century, despite the onset of declines in fertility. In France and the United States the birth rate was dropping early in the 19th century, and that of Ireland declined before the mid-1800's. As a general pattern, birth rates in northern and western Europe dropped during the fourth quarter of the 19th century. The pattern of decline spread to southern and eastern Europe only after 1900. Once the fertility decline began in a country, it continued without interruption, and the decline tended to be steeper where it began relatively late. Yet, despite the decreases in birth rate, fairly rapid, even if somewhat dampened, population growth continued in the West because death rates continued to decline. Appreciable decreases in growth rates occurred only among the nations most seriously affected by the Great Depression, during which both marriage and birth rates plummeted. In the aftermath of World War II, however, fertility increased to accelerate rates of total population growth. By the end of the 1950's, the postwar upsurge in birth rates had generally dissipated